The use of powerful anti-microbial agents in combination with improved living conditions dramatically increased our capacity to fight off previously lethal infections. However, in the last ten years there has been a dramatic escalation in antibiotic resistance among microorganisms1,2. Such resistance proliferates readily in the bacterial kingdom through gene transfer, making the spread of resistance hard to control. Although a multitude of efforts to improve the situation have been made it is clear that additional approaches for microbe control are required3. One obvious way to address this challenge is to develop novel anti-microbial drugs. This task is however arduous and most antibacterial agents that have reached the market during the last decade are based on previously characterized structures with known modes of action i.e. a bactericidal or static effect. Recent advances in combinatorial chemistry4-6, genomics7,8, and screening technologies9,10 however increase our capacity to identify novel bacterial targets and compounds that interfere with them. Combinatorial chemistry can indeed generate large numbers of diverse compounds but screening for novel agents from such libraries still mainly focus on agents that target microbial growth essentially in analogy with existing drugs11. An alternative is to identify compounds with a novel mode of action that target microbial virulence rather than growth12,13. Virulence includes events that enable the bacterium to enter the host, disarm the host's defence, multiply, and finally spread within the host or to a new host. Agents that target virulence are potentially effective antimicrobials but also apply less selective pressure for resistance. Moreover, compounds that perturb a virulence system can be employed as chemical probes to elucidate unknown features of bacterial virulence using a chemical genetics approach14,15.
Recent studies have revealed that various pathogenic bacteria use related virulence systems, findings that contradict the long held paradigm that each bacterium has a unique mode of action. The type III secretion system of Yersinia represents the archetype of one of these systems in which the bacteria adhere to eukaryotic cells and inject a set of bacterial effector proteins, Yops (yersinia outer proteins), that are capable of subverting the target cell16,17. This process involves a secretion of the Yops across the bacterial membranes and a subsequent translocation across the eukaryotic cell membrane. The genus Yersinia includes eleven known species of which Y. pestis, Y. speudotuberculosis, and Y. enterocolitica are pathogenic to humans of which Y. pestis, the causative agent of plague, is one of the most virulent bacteria known to man. These three species all share the tropism for lymphoid tissue and a capacity to evade the non-specific immune response. The processes of protein secretion and translocation represent attractive points of attack for novel antibacterial agents. The secretion apparatus is essential for the bacteria to evade the immune defence and it is possible that agents that inhibit the secretion can result in an antibacterial response without actually killing the bacteria. Moreover, several mammalian pathogens including Yersinia spp., Salmonella spp., Shigella flexneri, Pseudomonas aureginosa, enteropathogenic Escherichia coli, Chlamydia spp., and also plant pathogens like Xanthomonas campestris, Erwinia spp., Pseudomonas syringae, and Ralstonia solanacearum employ type III secretion systems that are crucial for virulence18,19. Some components of type III secretion systems in different species are interchangeable, which suggests evolutionary conservation and that data generated with one genus might also be valid for others. Thus, the type III secretion in gram negative bacteria is an important virulence system that constitutes an attractive drug target as well as a challenge for chemical genetics. The relevance of type III secretion in basic research and drug development is further stressed by the fact that multi-resistance strains have been found in Y. pestis20,21 and that Y. pestis is a potential weapon in biological warfare and bioterrorism22,23.
U.S. Pat. No. 6,136,542 discloses a method for screening agents that activate or inhibit type II secretion machinery in gram-negative bacteria. The method comprises exposing gram-negative bacterial cells to a sample of an agent to be screened, which cells contain a reporter gene, such as the luxAB gene, transcriptionally fused to a promoter of a gene activated or regulated by the type III secretion machinery, and detecting the presence or activity of the product of the reporter gene. The detection indicates whether the sample activates or inhibits type III secretion machinery. The gram-negative bacteria is selected from, i.a., Yersinia. 